Hollow fine SiO2 particles have an air gap in the interior of the SiO2 shell and have a high porosity, a low refractive index and a low dielectric constant and are thereby useful for prevention of reflection, an optical filter, a heat insulating material, a low dielectric constant material, drug delivery, etc. Particularly, they are known to be useful as an antireflection coating film material by virtue of their low refractive index owing to their hollow shape, and various processes for their production have been studied. Generally, such hollow fine SiO2 particles are obtained by removing only core fine particles of core/shell fine particles wherein the shell is SiO2 so that the SiO2 shell remains and the interior of the particles becomes hollow.
In the conventional method, one technical point is to determine which core fine particles are used and how they are removed by e.g. dissolution.
For example, in a case where an organic polymer is used as the core fine particles, the core fine particles are removed by thermal decomposition (Patent Document 1), and in a case where an inorganic compound is used, it is common to remove the core fine particles by dissolving them with an acid (Patent Documents 2 and 3).
However, the former method has such a restriction that thermal decomposition under high temperature conditions is required, and in a case where the substrate is an organic resin, the organic polymer core fine particles are preliminarily thermally decomposed and then dissolved in a solvent to prepare a coating fluid, such being troublesome.
On the other hand, according to studies by the present inventors, the latter method, which seems to be carried out more easily, was found to have such a problem that the ionic strength in a solution suddenly increases by ions generated by addition of an acid and ions generated from the dissolved core fine particles, thus lowering the stability of the solution, whereby hollow fine SiO2 particles are agglomerated in an uncontrollable state. If the hollow fine SiO2 particles are agglomerated in an uncontrollable state, the agglomerated particle size tends to be too large, thus impairing transparency of the resulting coating film. Accordingly, the concentration of the solution when the inorganic compound core fine particles are dissolved is restricted to a low concentration range, such being disadvantageous in productivity. Further, bothersome ultrafiltration is required to remove generated ions, and in addition, a very long time is required for filtration.
Further, to remove the core fine particles, requirement such as thermal decomposition at high temperature in the case of the former method and addition of a strong acid in the case of the latter method are required, but as described above, it is difficult to control the dissolution step, and the core fine particles will not completely be decomposed or removed by dissolution unless operation conditions are very precisely controlled, and some of the core fine particles remain, whereby no sufficient antireflection properties will be obtained. Further, in a case where the core fine particles contain a SiO2 component, the SiO2 remains, whereby the antireflection properties are insufficient. As described hereinafter, in the present invention, ZnO which is easily soluble is used as the core, and accordingly such remaining of the core material will not occur, whereby high antireflection properties can be obtained.    Patent Document 1: JP-A-6-142491 (Claims 1 to 4)    Patent Document 2: JP-A-2000-500113 (Claims 1 to 17)    Patent Document 3: JP-A-2001-233611 (Claims 1 to 11)